Engineers use compression (also called source coding or source encoding) to reduce the bit rate of digital video. Compression decreases the cost of storing and transmitting video information by converting the information into a lower bit rate form. Decompression (also called decoding) reconstructs a version of the original information from the compressed form. A “codec” is an encoder/decoder system.
Over the last two decades, various video codec standards have been adopted, including the H.261, H.262 (MPEG-2 or ISO/IEC 13818-2), H.263 and H.264 (AVC or ISO/IEC 14496-10) standards and the MPEG-1 (ISO/IEC 11172-2), MPEG-4 Visual (ISO/IEC 14496-2) and SMPTE 421M standards. More recently, the HEVC standard is under development. A video codec standard typically defines options for the syntax of an encoded video bitstream, detailing parameters in the bitstream when particular features are used in encoding and decoding. In many cases, a video codec standard also provides details about the decoding operations a decoder should perform to achieve correct results in decoding. Aside from codec standards, various proprietary codec formats define other options for the syntax of an encoded video bitstream and corresponding decoding operations.
Some types of parameters in a bitstream indicate information about reference pictures used during video encoding and decoding. A reference picture is, in general, a picture that contains samples that may be used for inter-picture prediction in the decoding process of other pictures. Typically, the other pictures follow the reference picture in decoding order and use the reference picture for motion-compensated prediction. In some video codec standards and formats, multiple reference pictures are available at a given time for use for motion-compensated prediction. Such video codec standards/formats specify how to manage the multiple reference pictures.
In general, a reference picture list (“RPL”) is a list of reference pictures used for motion-compensated prediction. In some video codec standards and formats, a reference picture set (“RPS”) is a set of reference pictures available for use in motion-compensated prediction at a given time, and an RPL is some of the reference pictures in the RPS. Reference pictures in an RPL are addressed with reference indices. A reference index identifies a reference picture in the RPL. During encoding and decoding, an RPS can be updated to account for newly decoded pictures and older pictures that are no longer used as reference pictures. Also, reference pictures within an RPL may be reordered such that more commonly used reference pictures are identified with reference indices that are more efficient to signal. In some recent codec standards, an RPL is constructed during encoding and decoding based upon available information about the RPS, modifications according to rules and/or modifications signaled in the bitstream. Signaling of modifications for an RPL can consume a significant amount of bits.